Maximize loss on non-target variable
You can maximize the loss by doing 1-loss (if the range of the loss is between 1 and 0. However, I am not sure if that would help. The network can just propagate the weights of the source classifier to always output the wrong answer. a better approach is to hand craft features of the document to make it less identifiable as the source. If the document have stated the source, remove it. This may help. Also, another way is to use documents from more sources so the network cannot use the source to classify the label. Hope I can help you.